1. Field of the Invention
The present invention relates to a display device and a driving method thereof, and more particularly, it relates to an organic light emitting diode (also referred to as “OLED,” hereinafter) display device, a display panel, and a driving method thereof.
2. Description of the Related Art
In general, an EL display device is a display device that electrically excites phosphorus organic components, and represents an image by voltage-programming or current-programming m×n numbers of organic light emitting pixels. As shown in FIG. 1, each of these organic light emitting pixels includes anode (indium tin oxide: ITO), organic thin film, and cathode (metal) layers. The organic thin film layer has a multi-layered structure including an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) so as to balance electrons and holes and thereby enhance efficiency of light emission. Further, the organic thin film includes an electron injection layer (EIL) and a hole injection layer (HIL).
Methods of driving the organic light emitting pixels can include a passive matrix method and an active matrix method. The active matrix method employs a thin film transistor (TFT). In the passive matrix method, an anode and a cathode are formed crossing each other, and a line is selected to drive the organic light emitting pixels. On the other hand, in the active matrix method, each indium tin oxide (ITO) pixel electrode (or anode) is coupled to the TFT and the light emitting pixel is driven in accordance with a voltage maintained by the capacitance of a capacitor coupled to a gate of the TFT. The active matrix method can also be classified into a voltage programming method and a current programming method depending on a type of signals transmitted to the capacitor so as to distinctively control the voltage applied to the capacitor.
FIG. 2 is an equivalent circuit diagram of a pixel circuit according to a conventional voltage-programming method.
Referring now to FIG. 2, a conventional organic EL display device employing the voltage-programming method supplies currents to an organic light emitting pixel or OLED through a transistor M coupled thereto for light emission, and the amount of current supplied to the OLED is adjusted by a data voltage applied through a switching transistor M2. Herein, a capacitor C1 is coupled between a source and a gate of the transistor M1 to maintain the amount of the data voltage applied during a predetermined time period.
When the transistor M2 is turned on, the data voltage is applied to the gate of the transistor M1, and a voltage of VGS between the gate and the source is charged to the capacitor C1. A current IOLED flows corresponding to the voltage of VGS, and the OLED emits light corresponding to the current IOLED.
Herein, the current flowing to the OLED is given as Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                                                                V                      TH                                                                                          )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          [                  Equation          ⁢                                          ⁢          1                ]            
where IOLED represents a current flowing to the OLED, VGS represents a voltage between the gate and the source of the transistor M1, VTH represents a threshold voltage of the transistor M1, VDATA represents a data voltage, and β represents a constant number.
As shown in Equation 1, the current corresponding to the data voltage is supplied to the OLED, and the OLED emits light corresponding to the current supplied thereto. Herein, the data voltage has multi-level values within a predetermined range to express gray scales.
However, a pixel circuit according to a conventional voltage-programming method has a problem in expressing high-level gray scales due to a deviation of a threshold voltage VTH at a driving transistor or a TFT and a mobility of a carrier. The deviation can result from a non-uniform manufacturing process of the TFT. For example, when a pixel circuit drives a TFT in a pixel by applying 3V thereto to express 8-bit gray scales (256 gray scales), a voltage should be applied to a gate of the TFT at an interval of less than 12 mV (=3V/256). However, it is difficult to express such a high gray scale in the case that the deviation of the threshold voltage VTH is 100 mV due to the non-uniform manufacturing process. Moreover, the deviation of the mobility of the carrier causes the value of β to be changed in Equation 1, and thus expressing the high level gray scale becomes even more difficult.
By contrast, although the amount of current and voltage supplied from a driving transistor to each of the pixels may not be uniform, the circuit of the pixels employing a current-programming method can still have a uniform panel as long as the currents supplied from a current source to the pixel circuit are uniform.
FIG. 3 shows an equivalent circuit diagram of a pixel circuit according to a conventional current-programming method.
As shown in FIG. 3, a transistor M1 is coupled to an OLED to supply a current for light emission, and the amount of the current is adjusted by a data current applied through a transistor M2.
Accordingly, when transistors M2 and M3 are turned on, a voltage corresponding to the data current IDATA is stored in a capacitor C1, and then the amount of current corresponding to the voltage stored in the capacitor C1 flows to the OLED so that the OLED can emit light. Herein, the current flowing to the OLED is given as Equation 2.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                                                                V                      TH                                                                                          )                            2                                =                      I            DATA                                              [                  Equation          ⁢                                          ⁢          2                ]            
where VGS represents a voltage between a gate and a source of a transistor M1, VTH represents a threshold voltage of the transistor M1, and β represents a constant number.
As shown in Equation 2, the current flowing throughout a panel can be uniform since the amount of the current IOLED flowing to the OLED and the amount of the data current IDATA are the same according to the conventional current-programming method. However, if a weak current (IDATA) flows to the OLED, it takes too much time to charge data lines. For instance, assume that the load of capacity in the data line is set to be 30 pF. In this case, it takes several milliseconds to charge the load of the capacity with data currents of several tens of nA to several hundreds of nA. However, line time is inefficient for fully charging the data line since it is limited to several μs.
On the other hand, if the amount of the current IOLED flowing to the OLED is increased to reduce time for charging the data line, brightness of all the pixels may be increased, thereby resulting in a decrease of image quality.